1. Field of the Invention
This invention relates to a pattern sensor for a sewing machine for maintaining correct alignment of a pattern on two layered sheets of cloth during sewing.
2. Prior Art
One type pattern sensor of a sewing machine is disclosed in U.S. Pat. No. 4,612,867. In this art, two sensors are provided facing each other with the cloths therebetween. Namely, one is provided over an upper cloth and the other is provided under a lower cloth. These two sensors respectively sense a pattern on each cloth at a predetermined position upstream of a stitching position.
When two sheets of cloth having patterns on their front surfaces are stitched up together so as to align their patterns by a sewing machine utilizing the above-mentioned prior art sensors, an operator is required to first visually align the patterns of the two sheets of cloth before the operator sets the cloths on the machine. Moreover, the operator must hold the cloths at a position upstream of the stitching position so as to align the edges of the two sheets of cloth during sewing. If the edges become unaligned, it becomes impossible to accurately sense the patterns, since each of the sensors respectively senses a pattern at a different position. Such incorrect sensing of patterns on the separate sheets of cloth makes it hard to correctly align the patterns. As a result, two sheets of cloth are sewn together with the patterns out of alignment. In order to avoid the disalignment of the edges, the sewing speed must be decreased so that the operator can accurately align the edges of the two sheets of cloth.
Two sheets of cloth are usually sewn with each of the fronts inside so that the fronts appear on a same side and the seams do not show. The sensors of the prior art, which are located over and under the cloths, therefore, sense the patterns from the back. In the case of sensing the patterns printed on the front, it is hard to accurately sense the patterns from the back, since the patterns sometimes do not appear clearly on the back. For solving the above-mentioned problem, it is possible to vertically dispose the sensors between the two sheets of cloth at a certain distance upstream of the stitching position. In this case, however, since the upper and the lower cloths are so separated significantly, it is difficult to handle the two sheets of cloth.
To cope with this, it is possible to dispose the two sensors horizontally so as to reduce the distance separating the two sheets of cloth. In this case, however, another problem will result. Since the sensors respectively sense patterns at separate positions, the pattern signals sent from the sensors must be corrected based on the positional difference of the sensors.